Stereo Audio Signal Encoder

1. A method comprising: receiving at least two audio channel signals; scalar quantizing a parameter to generate an index value wherein the parameter represents a difference between the at least two audio channel signals and comprises at least one of an interaural time difference or an interaural level difference; determining a frequency distribution of index values by maintaining a running count of index values, wherein the running count of index values associated with past indices are weighted downwards by a recurrence factor which is less than one; reordering the index value dependent on the frequency distribution of index values to generate a reordered index value; encoding the reordered index value dependent on an order position of the reordered index value; generating a single channel representation of the at least two audio channel signals dependent on the at least one parameter; and encoding the single channel representation.

2. The method as claimed in claim 1 , wherein scalar quantizing a parameter further comprises: ordering the scalar quantized output according to a predetermined map.

3. The method as claimed in claim 2 , wherein determining a frequency distribution of index values by maintaining a running count of index values where the past indices are weighted downwards by a recurrence factor further comprises: receiving a group of index values; increasing a count value associated with a first index value; and decreasing a count value associated with index values other than the first index value.

4. The method as claimed in claim 1 , wherein encoding the reordered index value dependent on an order position of the reordered index value comprises applying a Golomb-Rice encoding to the reordered index value dependent on the reordered index value order position.

5. A method comprising: decoding from a first part of a signal an index value of a parameter instance, wherein the parameter represents a difference between at least two audio channel signals and comprises at least one of an interaural time difference and an interaural level difference, and wherein the signal is an encoded multichannel audio signal; determining a frequency distribution of index values by maintaining a running count of index values, wherein the running count of index values associated with past indices are weighted downwards by a recurrence factor which is less than one; reordering the index values to generate a reordered index value, wherein the reordering is dependent on the frequency distribution of reordered index values; and dequantizing the reordered index value to generate the parameter.

6. The method as claimed in claim 5 , wherein decoding from a first part of a signal an index of a parameter comprises decoding a first part of a signal using a Golomb-Rice decoding.

7. The method as claimed in claim 6 , wherein determining the frequency distribution of the reordered index value comprises maintaining a count of the reordered index value for a group of the reordered index value.

8. The method as claimed in claim 5 , wherein the reordered index value comprises: determining an inverse ordering dependent on a decreasing occurrence order mapping for the frequency distribution of reordered index values; and applying the inverse ordering.

9. An apparatus comprising at least one processor and at least one memory including computer program code for one or more programs, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to: receiving at least two audio channel signals; scalar quantize a parameter to generate an index value wherein the parameter is a difference between the at least two channel audio signals comprising at least one of an interaural time difference and an interaural level difference; determine a frequency distribution of index values by maintaining a running count of index values , wherein the running count of index values associated with past indices are weighted downwards by a recurrence factor which is less than one; reorder the index value dependent on the frequency distribution of index values to generate a reordered index value; encode the reordered index value dependent on an order position of the reordered index value; generate a single channel representation of the at least two audio channel signals dependent on the at least one parameter; and encode the single channel representation.

10. The apparatus as claimed in claim 9 , wherein the apparatus caused to scalar quantize a parameter is further caused to: order the scalar quantized output according to a predetermined map.

11. The apparatus as claimed in claim 10 , wherein the apparatus caused to determine a frequency distribution of index values by maintaining a running count of index values where the past indices are weighted downwards by a recurrence factor further comprises: receive a group of index values; increase a count value associated with a first index value; and decrease a count value associated with index values other than the first index value.

12. The apparatus as claimed in claim 9 , wherein the apparatus caused to encode the reordered index value dependent on an order position of the reordered index value is further caused to apply a Golomb-Rice encoding to the reordered index value dependent on the reordered index value order position.

13. An apparatus comprising at least one processor and at least one memory including computer program code for one or more programs, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to: decode from a first part of a signal an index value of a parameter instance, wherein the parameter represents a difference between at least two audio channel signals and comprises at least one of an interaural time difference or an interaural level difference, and, wherein the signal is an encoded multichannel audio signal; determine a frequency distribution of index values by maintaining a running count of index values, wherein the running count of index values associated with past indices are weighted downwards by a recurrence factor which is less than one; reorder the index values to generate a reordered index value, wherein the reorder is dependent on the frequency distribution of reordered index values; and dequantize the reordered index value to generate the parameter.

14. The apparatus as claimed in claim 13 , where the apparatus caused to decode from a first part of a signal an index of a parameter comprises decoding a first part of a signal using a Golomb-Rice decoding.

15. The apparatus as claimed in claim 14 , wherein the apparatus caused to determine the frequency distribution of the reordered index value is further causes to maintain a count of the reordered index value for a group of the reordered index value.

16. The apparatus as claimed in claim 13 , wherein the apparatus caused to reorder index values is further caused to: determine an inverse ordering dependent on a decreasing occurrence order mapping for the frequency distribution of reordered index values; and apply the inverse ordering.